<?php
class MyTranslator implements PhpTal\TranslationService {
    …
    public function translate($key){
        $value = $this->_currentDomain[$key];

        // interpolate ${myvar} using context associative array
        while (preg_match('/\${(.*?)\}/sm', $value, $m)){
            list($src,$var) = $m;
            if (!array_key_exists($var, $this->_context)){
                $err = sprintf('Interpolation error, var "%s" not set',
                               $var);
                throw new Exception($err);
            }
            $value = str_replace($src, $this->_context[$var], $value);
        }

        return $value;
    }
    …
}
?>